1. Field of the Invention
This invention relates generally to a nuclear reactor core and more particularly, to a cooling arrangement for a reflector assembly of a core of a liquid metal-cooled fast breeder nuclear reactor.
2. Description of the Prior Art
A nuclear core of a liquid metal-cooled fast breeder reactor comprises a plurality of fuel assemblies, radial blanket assemblies (breeder assemblies), reflector assemblies and control rod assemblies. The fuel assemblies contain nuclear fuel which undergoes nuclear fission thereby producing heat which is eventually converted into commercial electrical energy. The radial blanket assemblies contain a fertile material which on capture of an excess neutron produced by the fission process, is converted into a fissile material which may then be used in another core of another reactor. The reflector assemblies serve as shielding and improve the nuclear performance of the core including the radial blanket assemblies by reflecting neutrons produced by the fission process back into the nuclear core. These neutrons would otherwise escape from the core and be wasted. Control or adjustment of the power output by the reactor and shutdown of the reactor is accomplished by control rod assemblies which function to poison the nuclear reaction by absorption of neutrons.
In the type of nuclear reactor considered herein, the heat produced by the fission process is removed by flowing a reactor coolant, such as liquid sodium, through the nuclear core. This coolant flow also removes heat from the core which is generated by neutron absorption and gamma heating within for example, the reflector assemblies. The reactor coolant flow is internal to all the assemblies contained within the nuclear core. The coolant flow is kept within the individual assemblies by flow barriers or "cans" comprising hexagonal containers which enclose each assembly of the core. This type of arrangement permits individual tailoring or adjusting of the coolant flow through the various assemblies in accordance with the particular cooling requirements of these individual assemblies. For example, the amount of heat generated in all of the reflector assemblies is less than one percent of the total reactor power; therefore, as compared to the fuel assemblies, a much lesser coolant flow rate is required to cool the reflector assemblies. Nuclear cores using the aforementioned reactor coolant flow arrangement are generally designated as closed cores. By way of contrast, most pressurized water reactors use an open core arrangement which allows reactor coolant to flow indiscriminantly both through and between the various core assemblies.
A number of distinct advantages are gained from the use of a closed core in a nuclear reactor. This is especially so for a liquid metal-cooled fast breeder reactor. First, a significant improvement in reactor performance is realized because of the elimination or minimization of reactor coolant flow which bypasses the nuclear core. Second, it enhances the structural integrity of the nuclear core by reducing the probability of failure propagation between fuel assemblies. However, some disadvantages also result from a closed core cooling arrangement. One major disadvantage is inherent design deficiencies of the orifices which are required to reduce the rate of coolant flow through reflector assemblies as compared to the coolant flow rate through the fuel assemblies. The prior art solution was to design orifice plates to fit within the individual assemblies which result in very high pressure drops to achieve very low flow rates through the assemblies. Since the velocity of the coolant flow through such orifice plates is very high, flow cavitation is possible and the orifices are subjected to wear by erosion. The erosion makes it extremely difficult to assure adequate performance of such orifice plates over the long residence time of the reflector assemblies in the nuclear core. Further, since the performance of orifices deteriorates rapidly with off design conditions, operation of the nuclear reactor at low power levels and low flow rates causes severe maldistribution of the flow through the reflector assemblies which in turn, results in ineffective cooling of the reflector assemblies.